Systems for Printing Three-Dimensional Objects

ABSTRACT

The present disclosure provides a system for printing at least a portion of a three-dimensional (3D) object. The system may comprise a source of at least one feedstock, a support for supporting at least a portion of the 3D object, a feeder for directing at least one feedstock from the source towards the support, and a power supply for supplying electrical current. The system may comprise a controller operatively coupled to the power supply. The controller may receive a computational representation of the 3D object. The controller may direct the at least one feedstock through a feeder towards the support and may direct electrical current through the at least one feedstock and into the support. The controller may subject such feedstock to Joule heating such that at least a portion of such feedstock may deposit adjacent to the support, thereby printing the 3D object in accordance with the computational representation.

TECHNICAL FIELD

In various embodiments, the present invention relates to additivemanufacturing techniques such as three-dimensional (3D) printing, and inparticular to the additive manufacturing of metallic objects.

BACKGROUND

Additive manufacturing techniques such as 3D printing are rapidly beingadopted as useful techniques for a host of different applications,including rapid prototyping and the fabrication of specialty components.To date, most additive manufacturing processes have utilized polymericmaterials, which are melted, layer-by-layer, into specified patterns toform 3D objects. The additive manufacturing of metallic objectspresented additional challenges, but techniques have been more recentlydeveloped to address these challenges.

Existing technologies for the additive manufacture of metal structuresmay generally be classified in three categories: laser sintering,adhesive bonding followed by sintering, and molten metal deposition. Thetwo sintering technologies use a bed of metal powder in the build area,and the powder particles are selectively joined to one another to formthe desired pattern. When one layer is completed, more metal powder isspread over the first layer, and powder particles are joined to theprevious layer in the pattern required for that layer. The processcontinues with fresh powder spread over the entire surface of the buildarea and then selectively joined, building the desired structure layerby layer. The finished part is retrieved from inside the powder bed, andthe powder is then emptied from the build area to begin the next part.

However, the use of metal powder as a raw material can be problematicfor several reasons. Metal powder is expensive to produce, and generallyis more expensive than a wire made from the same material for the samevolume of material. Metal powders are difficult and dangerous to handle.For example, metal powder that is spilled may form dust in the air thatis dangerous to inhale, and such dust may even be an explosion risk. Inaddition, the amount of powder required for conventional additivemanufacturing technologies is many times greater than that required tomake the part, as the entire build area must be filled with powder. Thisincreases the cost of the process and leads to attrition and waste ofpowder, which may not be readily reused. Conventional powder-basedprocesses are also very slow because the spreading of concurrent layersof powder typically must be done precisely to the required layerthickness and must be done across the entire build area for each layer.

Laser sintering uses a high power laser as the source of heat to fuseparticles. Lasers have many safety risks, especially at the powersrequired for fusing metals. Using lasers as a source of heat causesissues because the particles must be heated top down to add enough heatto fuse them to the previous layer. Such top-down heating requires muchmore heat than would be needed if the heat was applied directly to thejoining surfaces, which slows down the overall process and causes theexcess heat to be dissipated into the powder bed. Because of this, thereis the danger of unwanted sintering particles in the area around thatwhich the laser is heating. Therefore, the process requires the use ofmetals and alloys that have poor heat conduction.

Adhesive bonding uses glue to join adjacent powder particles instead ofdirectly fusing the particles by laser energy, but the process isotherwise similar. Glue is selectively sprayed to form a pattern, andpowder is added layer by layer to form the structure. To make amechanically sound metal part, the structure generally must be removedfrom the powder bed and placed in a furnace to sinter the bonded metalpowders. The sintering multiplies the complexity of the process and wellas the time required to produce parts.

In molten-metal deposition techniques, heat to liquefy the metal isderived from plasma or electric arc. The molten metal is then sprayed inthe pattern desired to form a structure by building layers as the metalcools. The resolution achieved by spraying metal is generally poorcompared to other processes, to the extent that hybrid machines havebeen developed to deposit metal, allow it to cool, and then use amilling tool to machine it to size. The speed of the process is slowbecause sufficient time must be allowed to cool the underlying layerbefore it can be built upon, as the heat generated by the plasma orelectric arc are very high. It is further slowed by the machiningprocess if good resolution is required.

In view of the foregoing, there is a need for improved additivemanufacturing techniques for the fabrication of metallic parts that donot utilize metal powders as raw materials, do not generate excessiveheat, and do not require time-consuming and uneconomical sintering stepsfor solidification.

SUMMARY

In accordance with various embodiments of the present invention, metalobjects are fabricated layer by layer in a controlled manner utilizingmetal wire as feedstock, enabling the manufacture of 3D structures.Embodiments of the invention only utilize as much feedstock as requiredto form the object being fabricated, eliminating most (if not all) ofthe waste and/or recovery processes associated with powder-basedtechniques. The metallic wire feedstock is more easily handled andenables faster fabrication, as it is deployed only at the exact pointswhere the solid structure is being fabricated. The wire is heated uponcontact with the fabrication platform or a previous layer of thestructure being fabricated via a pulse of electric current, forming amolten droplet (or “particle”) at the point of contact. The moltendroplets adhere in place, enabling the layer-by-layer fabrication of thepart. Advantageously, only the single melting/deposition step isrequired, obviating the need for separate sintering steps to bond themetal particles together. In addition, current is typically only appliedto the wire upon contact with the fabrication platform or a previouslayer of the structure being fabricated, thereby minimizing heating ofthe wire (and the structure being fabricated) and preventing formationof electrical arcs at the wire tip.

Embodiments of the present invention have the advantage that heat isgenerated at the point of contact between adjacent particles (i.e.,between the tip of the wire feedstock and the fabrication platform or aprevious layer of the structure being fabricated), exactly where theheat is required for fusion. This allows much lower heat input than thatutilized in laser-heating techniques. The lower heat input enablesfaster overall processing, no risk of unwanted heating of surroundingparticles, and the use of many different metals and alloys. It alsoreduces safety concerns, and the build area typically is maintained at alower temperature.

Embodiments of the present invention solve the problems inherent toexisting approaches by leveraging knowledge of established gas metalsarc welding (GMAW), resistive spot welding (RSW), and computer-aidedmanufacturing (CAM) technologies. Embodiments of the invention utilizeinert gas shielding and a fine metal wire electrode as both an electrodeand source of metal feedstock (similarly to GMAW), an electric currentthat heats and melts the feed metal and base metal due to contactresistance (similarly to RSW), and can control the motion of the metalwire electrode/feedstock in three dimensions through acomputer-controlled interface, allowing for deposition of material inthe desired shape (similarly to CAM). These features enable theproduction of 3D metal structures using any of a variety of metals andmetal alloys with minimal safety concerns at low cost.

In an aspect, embodiments of the invention feature a method oflayer-by-layer fabrication of a three-dimensional metallic structureupon an electrically conductive base. A first layer of the structure isformed by depositing a plurality of metal particles onto the base. Eachmetal particle is deposited by (i) disposing a metal wire in contactwith the base, and (ii) passing an electrical current through the metalwire and the base. A portion of the metal wire melts to form the metalparticle on the base. One or more subsequent layers of the structure areformed by depositing pluralities of metal particles over the first layerof the structure. Each metal particle is deposited by (i) disposing themetal wire in contact with a previously deposited metal particle, and(ii) passing an electrical current through the metal wire, thepreviously deposited metal particle, and the base. A portion of themetal wire melts to form the metal particle on the previously depositedmetal particle.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. A gas may be flowed over at least atip of the wire during deposition of the metal particles. The gas mayreduce or substantially prevent oxidation of the metal particles duringdeposition. The gas may increase a cooling rate of the metal particlesduring deposition. After deposition of each metal particle, a relativeposition of the metal wire and the base may be changed with one or moremechanical actuators (e.g., stepper motors, solenoids, etc.). The metalwire may include, consist essentially of, or consist of stainless steel,copper, and/or aluminum. A porosity of at least a portion of thestructure may be controlled by (i) altering a spacing between adjoiningcontact points between the metal wire and the base or underlyingparticles, and/or (ii) altering a magnitude of the current appliedbetween the metal wire and the base. A computational representation of athree-dimensional structure may be stored. Sets of data corresponding tosuccessive layers may be extracted from the computationalrepresentation, and each of the forming steps may be performed inaccordance with the data. A size of at least one metal particle may beselected by controlling a speed of retraction of the metal wiretherefrom (e.g., during and/or after deposition). An outer portion ofthe metal wire may be removed before the metal wire is melted to form atleast one of the metal particles. An amount of metal wire utilized toform the first layer and the one or more subsequent layers of thestructure may be tracked and/or stored. The metal particles may beformed in response to heat arising from, at least in part (e.g.,substantially entirely due to), contact resistance at the tip of thewire (i.e., resistance resulting from contact between the tip of thewire and an underlying structure, e.g., the base or an underlyingparticle).

In another aspect, embodiments of the invention feature an apparatus forthe layer-by-layer fabrication of a three-dimensional metallic structurefrom particles formed by melting a metal wire. The apparatus includes orconsists essentially of an electrically conductive base for supportingthe structure during fabrication, a wire-feeding mechanism fordispensing wire over the base, one or more mechanical actuators forcontrolling a relative position of the base and the wire-feedingmechanism, a power supply for applying a current between the wire andthe base sufficient to cause the wire to release a metal particle (e.g.,via heat arising from contact resistance between the wire and an objectin contact therewith, e.g., the base), and circuitry for controlling theone or more actuators and the power supply to create thethree-dimensional metallic structure on the base from successivelyreleased metal particles.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The circuitry may include or consistessentially of a computer-based controller for controlling the one ormore mechanical actuators and/or the power supply. The computer-basedcontroller may include or consist essentially of a computer memory and a3D rendering module. The computer memory may store a computationalrepresentation of a three-dimensional structure. The 3D rendering modulemay extract sets of data corresponding to successive layers from thecomputational representation. The controller may cause the mechanicalactuators and the power supply to form successive layers deposited metalparticles in accordance with the data. Metal wire may be disposed withinthe wire-feeding mechanism.

In yet another aspect, embodiments of the invention feature a method oflayer-by-layer fabrication of a three-dimensional metallic structureupon an electrically conductive base. A sacrificial raft structure isformed by depositing a plurality of metal particles onto the base. Eachmetal particle is deposited by (i) disposing a first metal wire incontact with the base, and (ii) passing an electrical current throughthe first metal wire and the base. A portion of the first metal wiremelts to form the metal particle on the base. A first layer of thestructure is formed by depositing a plurality of metal particles ontothe sacrificial raft structure. Each metal particle is deposited by (i)disposing a second metal wire in contact with the sacrificial raftstructure, and (ii) passing an electrical current through the secondmetal wire, the sacrificial raft structure, and the base. A portion ofthe second metal wire melts to form the metal particle on thesacrificial raft structure. One or more subsequent layers of thestructure are formed by depositing pluralities of metal particles overthe first layer of the structure. Each metal particle is deposited by(i) disposing the second metal wire in contact with a previouslydeposited metal particle, and (ii) passing an electrical current throughthe second metal wire, the previously deposited metal particle, thesacrificial raft structure, and the base. A portion of the second metalwire melts to form the metal particle on the previously deposited metalparticle.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The density and/or the porosity of thesacrificial raft structure may be less than that of the structure. Thesacrificial raft structure may define one or more openings therethrough.The sacrificial raft structure may include, consist essentially of, orconsist of a plurality of layers. A thickness of at least one of thelayers of the sacrificial raft structure may be greater than a thicknessof at least one of the layers of the structure. A thickness of at leastone of the layers of the sacrificial raft structure may be greater thana thickness of all of the layers of the structure. A thickness of abottommost layer of the sacrificial raft structure (i.e., the layer ofthe sacrificial raft structure directly in contact with the base) may begreater than a thickness of at least one of, or even all of, the layersof the structure. After fabrication of the structure, the sacrificialraft structure may be removed from the base, and at least a portion ofthe structure may remain on the sacrificial raft structure. After thesacrificial raft structure is removed from the base, the sacrificialraft structure may be separated from the structure. The first and secondmetal wires may include, consist essentially of, or consist of differentmaterials (e.g., different metals). The first and second metal wires mayinclude, consist essentially of, or consist of the same material (e.g.,the same metal). The metal particles may be formed in response to heatarising from, at least in part (e.g., substantially entirely due to),contact resistance at the tip of the wire (i.e., resistance resultingfrom contact between the tip of the wire and an underlying structure,e.g., the base, the raft, or an underlying particle).

These and other objects, along with advantages and features of thepresent invention herein disclosed, will become more apparent throughreference to the following description, the accompanying drawings, andthe claims. Furthermore, it is to be understood that the features of thevarious embodiments described herein are not mutually exclusive and mayexist in various combinations and permutations. As used herein, theterms “approximately” and “substantially” mean±10%, and in someembodiments, ±5%. The term “consists essentially of” means excludingother materials that contribute to function, unless otherwise definedherein. Nonetheless, such other materials may be present, collectivelyor individually, in trace amounts. For example, a structure consistingessentially of multiple metals will generally include only those metalsand only unintentional impurities (which may be metallic ornon-metallic) that may be detectable via chemical analysis but do notcontribute to function.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1 is a schematic of an additive manufacturing apparatus inaccordance with various embodiments of the invention;

FIGS. 2A-2F are schematics of the deposition of metallic particlesduring the fabrication of a three-dimensional object in accordance withvarious embodiments of the invention;

FIG. 3 is a schematic of a printed three-dimensional object havingregions of different particle resolutions in accordance with variousembodiments of the invention;

FIG. 4A is a schematic of particles printed with low porosity inaccordance with various embodiments of the invention;

FIG. 4B is a schematic of particles printed with high porosity inaccordance with various embodiments of the invention;

FIGS. 5A-5C schematically depict deposition of a particle from a wire inaccordance with various embodiments of the invention;

FIGS. 5D-5F schematically depict particles of different sizes depositedvia use of different wire-retraction rates in accordance with variousembodiments of the invention;

FIG. 6 is a schematic of a mechanical wire-tracking system in accordancewith various embodiments of the invention;

FIG. 7 is a schematic of an optical wire-tracking system in accordancewith various embodiments of the invention;

FIG. 8 is a schematic of an anti jamming mechanism in accordance withvarious embodiments of the invention;

FIGS. 9A and 9B are schematic plan views of sacrificial structuresprinted between the baseplate and a desired printed part in accordancewith various embodiments of the invention;

FIG. 9C is a schematic cross-sectional view of a part printed on asacrificial structure on a baseplate in accordance with variousembodiments of the invention;

FIG. 10A is a schematic illustration of removal of a sacrificialstructure and printed part thereover from a baseplate in accordance withvarious embodiments of the invention; and

FIG. 10B is a schematic illustration of removal of a sacrificialstructure from a printed part in accordance with various embodiments ofthe invention.

DETAILED DESCRIPTION

In accordance with embodiments of the invention, 3D metal structures maybe fabricated layer-by-layer using an apparatus 100, as shown in FIG. 1.Apparatus 100 includes a mechanical gantry 105 capable of motion in oneor more of five or six axes of control (e.g., one or more of the XYZplanes) via one or more actuators 110 (e.g., motors such as steppermotors). As shown, apparatus 100 also includes a wire feeder 115 thatpositions a metal wire 120 inside the apparatus, provides an electricalconnection to the metal wire 120, and continuously feeds metal wire 120from a source 125 (e.g., a spool) into the apparatus. A baseplate 130 isalso typically positioned inside the apparatus and provides anelectrical connection; the vertical motion of the baseplate 130 may becontrolled via an actuator 135 (e.g., a motor such as a stepper motor).An electric power supply 140 connects to the metal wire 120 and thebaseplate 130, enabling electrical connection therebetween. The motionof the gantry 105 and the motion of the wire feeder 115 are controlledby a controller 145. The application of electric current from the powersupply 140, as well as the power level and duration of the current, iscontrolled by the controller 145.

The computer-based controller 145 in accordance with embodiments of theinvention may include, for example, a computer memory 150 and a 3Drendering module 155. Computational representations of 3D structures maybe stored in the computer memory 150, and the 3D rendering module 155may extract sets of data corresponding to successive layers of a desired3D structure from the computational representation. The controller 145may control the mechanical actuators 110, 135, wire-feeding mechanism115, and power supply 140 to form successive layers deposited metalparticles in accordance with the data.

The computer-based control system (or “controller”) 145 in accordancewith embodiments of the present invention may include or consistessentially of a general-purpose computing device in the form of acomputer including a processing unit (or “computer processor”) 160, thesystem memory 150, and a system bus 165 that couples various systemcomponents including the system memory 150 to the processing unit 160.Computers typically include a variety of computer-readable media thatcan form part of the system memory 150 and be read by the processingunit 160. By way of example, and not limitation, computer readable mediamay include computer storage media and/or communication media. Thesystem memory 150 may include computer storage media in the form ofvolatile and/or nonvolatile memory such as read only memory (ROM) andrandom access memory (RAM). A basic input/output system (BIOS),containing the basic routines that help to transfer information betweenelements, such as during start-up, is typically stored in ROM. RAMtypically contains data and/or program modules that are immediatelyaccessible to and/or presently being operated on by processing unit 160.The data or program modules may include an operating system, applicationprograms, other program modules, and program data. The operating systemmay be or include a variety of operating systems such as MicrosoftWINDOWS operating system, the Unix operating system, the Linux operatingsystem, the Xenix operating system, the IBM AIX operating system, theHewlett Packard UX operating system, the Novell NETWARE operatingsystem, the Sun Microsystems SOLARIS operating system, the OS/2operating system, the BeOS operating system, the MACINTOSH operatingsystem, the APACHE operating system, an OPENSTEP operating system oranother operating system of platform.

Any suitable programming language may be used to implement without undueexperimentation the functions described herein. Illustratively, theprogramming language used may include assembly language, Ada, APL,Basic, C, C++, C*, COBOL, dBase, Forth, FORTRAN, Java, Modula-2, Pascal,Prolog, Python, REXX, and/or JavaScript for example. Further, it is notnecessary that a single type of instruction or programming language beutilized in conjunction with the operation of systems and techniques ofthe invention. Rather, any number of different programming languages maybe utilized as is necessary or desirable.

The computing environment may also include other removable/nonremovable,volatile/nonvolatile computer storage media. For example, a hard diskdrive may read or write to nonremovable, nonvolatile magnetic media. Amagnetic disk drive may read from or writes to a removable, nonvolatilemagnetic disk, and an optical disk drive may read from or write to aremovable, nonvolatile optical disk such as a CD-ROM or other opticalmedia. Other removable/nonremovable, volatile/nonvolatile computerstorage media that can be used in the exemplary operating environmentinclude, but are not limited to, magnetic tape cassettes, flash memorycards, digital versatile disks, digital video tape, solid state RAM,solid state ROM, and the like. The storage media are typically connectedto the system bus through a removable or non-removable memory interface.

The processing unit 160 that executes commands and instructions may be ageneral-purpose computer processor, but may utilize any of a widevariety of other technologies including special-purpose hardware, amicrocomputer, mini-computer, mainframe computer, programmedmicro-processor, micro-controller, peripheral integrated circuitelement, a CSIC (Customer Specific Integrated Circuit), ASIC(Application Specific Integrated Circuit), a logic circuit, a digitalsignal processor, a programmable logic device such as an FPGA (FieldProgrammable Gate Array), PLD (Programmable Logic Device), PLA(Programmable Logic Array), RFID processor, smart chip, or any otherdevice or arrangement of devices that is capable of implementing thesteps of the processes of embodiments of the invention.

Embodiments of the invention form metal structures via metal particlesformed at the molten tip of a metal wire, as shown in FIGS. 2A-2F. Asshown, the formation of the desired 3D structure typically begins withthe deposition of a single particle 200 melted from the wire 120 ontothe baseplate 130. The particle 200 and subsequent particles may haveany morphology but may be considered to be substantially spherical.Additional particles 205, 210 are deposited one by one adjacent topreviously deposited particles, and the heat from the formation of eachnew particle partially melts the adjacent particles and fuses themtogether. Once all of the particles that need to be adjacent to oneanother on a single layer for the desired structure have been deposited,deposition of particles 215, 220, 225 begins one by one on top of theprevious layer of fused particles 200, 205, 210. Deposition continues inthis manner, layer by layer, until the entire structure is completed.Each layer of the structure may be composed of a different number ofparticles, depending on the desired shape of the structure, andparticles in an overlying layer need not be (but may be, in variousembodiments) deposited directly on top of a particle of an underlyinglayer.

The diameter of the particles will typically determine the height ofeach layer, and as such may at least in part dictate the resolution atwhich structures may be formed. The diameter of the particles may bechanged by changing the diameter of the metal wire 120, as well as thedeposition parameters (e.g., current level), and thus the resolution ofthe structure may be controlled dynamically during the process. Ingeneral, higher resolution will increase the time required to form thestructure, and lower resolution will decrease it. Therefore, sections of3D structures may be fabricated with high resolution to hold a tightmechanical tolerance or to be more visually appealing, and otherssections may be fabricated at low resolution to increase the speed ofdeposition, as shown in FIG. 3. FIG. 3 depicts a printed structure 300composed of a low-resolution portion 305 at least partially surroundedby a high-resolution portion 310. As shown, the low-resolution portion305 includes or consists essentially of multiple larger particles 315,while high-resolution portion 310 includes or consists essentially ofmultiple smaller particles 320. The portions 305, 310 may include pores325 between particles that result from empty space remaining betweenparticles during melting thereof.

The porosity of the fabricated 3D structure may be determined, at leastin part, by the spacing and/or extent of fusion between adjacentparticles, as shown in FIGS. 4A and 4B. FIG. 4A depicts two particlesfused closely together, resulting in a smaller porosity signified bysmaller porous region 400 (which may, in a completed part, be at least aportion of a pore therewithin), and FIG. 4B depicts two particles fusedtogether to a lesser extent, resulting in higher porosity signified by alarger porous region 410. Deposition parameters may be varied todetermine the degree of fusion between particles, mainly through theamount of heat generated during deposition. If heat is increased, fusionbetween particles will be greater, and porosity will generally be lower.If enough heat is generated, the resulting structure may havesubstantially no porosity, which may be preferred to achieve specificmechanical properties. Conversely, less heat will cause less fusion, andporosity will be higher. A more porous structure will typically have alower weight than a fully dense structure. Since the amount of heat maybe controlled dynamically during deposition, sections of the 3Dstructure may be intentionally made more porous than other sections. Forexample, a porous filter may be contained in an internal passage of alarger 3D structure. In general, the application of less heat willrequire less time, so the speed of deposition may be increased ifporosity is desired or may be tolerated in sections of the structure.Materials with high porosity typically have low tensile strength but mayachieve good compressive strength. Structures may be designed so thatareas in compressive loading may be produced with some porosity, leadingto faster deposition speed, and also lower weight of the finishedstructure.

In accordance with embodiments of the present invention, metal particlesare formed by melting the tip of the metal wire 120 with electriccurrent. The wire 120 may have a substantially circular cross-section,but in other embodiments the wire 120 has a cross-section that issubstantially rectangular, square, or ovular. The diameter (or otherlateral cross-sectional dimension) of the metal wire 120 may be chosenbased on the desired properties of deposition, but generally may bebetween approximately 0.1 mm and approximately 1 mm. The metal wire 120is one electrode, and the metallic baseplate 130 of the apparatus 100 isthe other electrode, as shown in FIG. 1. When the wire 120 is inphysical contact with the baseplate 130 , the two are also in electricalcontact. There is an electrical resistance between the wire 120 andbaseplate 130 (i.e., contact resistance) due to the small surface areaof the fine wire 120 and the microscopic imperfections on the surface ofthe baseplate 130 and the tip of the wire 120. The contact resistancebetween the wire 120 and baseplate 130 is the highest electricalresistance experienced by an electric current that is passed between thetwo electrodes (i.e., the wire 120 and baseplate 130), and the localarea at the contact point is heated according to Equation 1 (i.e.,Joule's First Law).

Q=I ² ×R×t   Equation 1

The heat generated (Q) is in excess of the heat required to melt the tipof the wire 120 into a particle and to fuse the particle to adjacentparticles. The heat is determined by the amount of current passed (I),the contact resistance between the wire 120 and baseplate 130 (R), andthe duration of the application of current (t). (Thus, embodiments ofthe present invention form particles without use or generation ofelectrical arcs and/or plasma, but rather utilizecontact-resistance-based melting of the wire.) Current and time (I andt) may be controlled during the process via controller 145 and powersupply 140, and in various embodiments of the invention, a high currentis utilized for a short duration (as opposed to a lower current for alonger duration) to increase the speed of deposition. The requiredcurrent and duration depends on the desired deposition properties, butthese may generally range from approximately 10 Amperes (A) toapproximately 1000 A and approximately 0.01 seconds (s) to approximately1 s. After the first layer of fused particles is completed, the previouslayer of particles, which are in electrical contact with the baseplate130, act as the second electrode. As the process proceeds, one electrode(the metal wire 120) is consumed as metal from the tip of the wire 120is utilized to form the particle.

The use of a consumable metal wire as an electrode is similar to GMAW,in that the wire feedstock may be stored on large spools and feedcontinuously to continue the deposition process. Thus, there are manymetal and metal alloy wires that are readily available at low cost. Thedevices and techniques for the mechanical motion of feeding the wire andmaking electrical contact between the wire and the power supply are alsoknown to those of skill in the art. In order to protect the depositedmetal from oxidation, an inert gas (such as Ar) or semi-inert gas (suchas N₂ or CO₂) may be flowed over the area around the metal wireelectrode to displace oxygen. For example, gas may be flowedcontinuously at a rate of, e.g., approximately 0.7 m³/hr during thedeposition process when the metal is at high temperature or is molten.Advantageously, gas flow rates may be increased beyond what is requiredto provide a shielding effect to increase the rate at which depositedmetal cools. Cooling rate may also affect the resulting mechanicalproperties of the metal, and with dynamic control during deposition,sections of the structure may be fabricated with different mechanicalproperties. For example, a high cooling rate may be used on the surfaceof a structure to increase hardness and wear resistance, while a slowercooling rate may be used on the interior to maintain ductility andstrength.

In accordance with embodiments of the invention, the material for thebaseplate electrode 130 is selected for good electrical conductivity andcompatibility with the metal that is being deposited. The baseplate 130is typically non-consumable and thus is not damaged and need not bereplaced during normal operation. The baseplate material may be chosento allow weak adhesion of the deposited metal to it, so that the firstlayer of deposited metal will hold the structure firmly in place on thebaseplate 130 during further deposition. For example, if the depositedmetal is steel, copper or aluminum may be appropriate materials for thebaseplate 130. Copper and aluminum have a high electrical conductivity,will not alloy with steel and change the composition of the depositedmetal, and have good thermal conductivity so heat generated at thedeposition area may be quickly conducted away, and there is no danger ofmelting the baseplate 130. The surface finish of the baseplate 130 maybe slightly rough, so that the metal of the first layer melts into thefine surface features (e.g., scratches) of the baseplate 130 and allowsfor weak adhesion. The surface finish of the baseplate 130 may be chosento give the appropriate amount of adhesion so the structure is heldfirmly during deposition, but that a reasonable force may be used toremove the finished structure from the baseplate 130 at the end ofdeposition. The baseplate 130 may be made easily replaceable so that itmay be changed to an appropriate material for the desired depositionmetal.

The morphology of the deposited particles may be controlled through thediameter of the metal wire 120, as well as the deposition parameters.The diameter of the deposited particle will typically be roughly thesame diameter as the wire 120. The diameter of the particle may beincreased by feeding additional wire 120 into the particle while it isstill molten. The shape of the top of the particle may be influenced bythe retraction of the wire 120 while the particle is still molten, forexample, where the top of the particle may be drawn into a peak via wireretraction. If the particle is allowed to partially cool, the wire 120may be used to push the top of the particle to flatten the particle.These manipulations of the particle morphology may be used to change theporosity of the structure.

Similarly, retraction of the still molten wire tip from the previouslydeposited particle may be used to control the morphology of the tip ofthe wire 120, as illustrated in FIGS. 5A-5C. In various embodiments ofthe invention, if the wire 120 is retracted quickly, the tip will bedrawn into a sharp point. FIG. 5A depicts the initial formation of aparticle 500 melting from the tip of wire 120. In FIG. 5B, the wire 120is retracted from the particle 500, which is still at least partiallymolten. As shown, the tip of the wire 120 begins to neck down,decreasing its diameter. FIG. 5C illustrates the sharp tip 510 of thewire 120 after full retraction and separation from the particle 500. Thespeed to retraction may thus be used to control the diameter of the tipof the wire 120. Since the diameter at the tip is the effective diameterof the wire 120 for the next deposition, this controlled necking may beused to deposit particles with a diameter smaller than the bulk wirediameter. In this manner, higher resolution deposition is possible withlarger wire diameters. FIGS. 5D-5F illustrate different sized particles500 that may be deposited using the same wire via control of the wire'sretraction speed when depositing the previous particle.

Control of the application of electric current may be used to influencethe deposition of particles. Open-loop control of the applied current isenabled via choosing the desired intensity of power along with theduration prior to deposition. The intensity level may be calibrated toachieve a specific voltage or current at a constant contact resistance.However, the contact resistance may vary at each deposition site, aswell as vary during the particle deposition itself. Open-loop controlmay therefore result in the application of too much or too little heatduring deposition, and the fusion between particles may be affected.With proper calibration, open-loop control may be used successfully fordeposition. In other embodiments of the invention, closed-loop controlis used. In closed-loop control, the voltage and current are measuredduring deposition, and the contact resistance may be calculatedaccording to Equation 2 (i.e., Ohm's Law).

R=V/I   Equation 2

Because the contact resistance is calculated dynamically, the power ofthe applied electric current may be precisely controlled, thus resultingin the exact amount of heat being applied during deposition to achievethe desired deposition parameters and/or particle characteristics. Asmall AC current on the order of 1 mV to 100 mV may be applied inaddition to the DC current of the deposition circuit to determine theimpedance response of the system. The impedance may also be measureddynamically and used for feedback control. Closed-loop control maybeneficially eliminate failed parts due to incomplete fusion ofparticles and minimize heat input into the structure during deposition.

In addition to the data that may be measured from the electric circuitof the deposition (i.e., the circuit formed by the baseplate 130 andwire 120 via controller 145 and power supply 140), additional sensorsmay be utilized to gather complementary data. Temperature measurementsof the deposition site on the baseplate 130 or other points on theprinted part or apparatus 100 may be measured using contact sensors suchas thermocouples or thermistors, and non-contact methods such asinfrared (IR) sensors and optical pyrometry. Temperature data may thenbe used by the system control loop to ensure the desired depositionparameters.

Other sensors may be used to detect the build surface, i.e., thebaseplate 130 or the previously deposited layer of particles of the partbeing printed. Sonar or capacitive response systems may be used to mapthe surface and detect any areas that are not in specification, allowingfor corrective action (e.g., rework such as additional particledeposition in areas having high porosity or missing material). All thedata collected for feedback control may also be logged and then analyzedat the network level to develop automatic calibration processes toimprove the function of any connected apparatus 100.

To take advantage of the particle-by-particle deposition mechanism inembodiments of the present invention, the design process may be tailoredto make use of a voxel system. The 3D rendering module 155 may assignproperties to certain sections of the part based the depositionparameters desired using, e.g., computer-aided design (CAD) software.For example, if an internal section of a part should be porous to act asa filter, that section in the CAD design may be selected, and the usermay assign values to parameters such as the percent porosity desired. Intandem with the voxel-based extension for the 3D rendering module 155,computer-aided manufacturing software may be utilized to translate thedesired voxel properties into the toolpath and deposition parametersrequired to produce the user's CAD design.

Another example of a voxel-based design is the design of a heat sink. Inthe CAD design utilized by the 3D rendering module 155, the user mayspecify properties such as the material and density to direct heatthrough a specific area of the part. This concept may be used to keepheat-sensitive areas of the same part cool, without having to make thepart from multiple pieces or via multiple different depositions. Thevoxel-based design system may also be leveraged with control of surfacetextures of either external or internal surfaces. A surface mayintentionally be made with a very high surface area to give a part ahigh-friction surface, a highly radiant surface to cool moreeffectively, give an electrode higher conductivity, or allow forenhanced adhesion of a surface coating.

To deposit particles in precise locations, the metal wire electrode 120and baseplate 130 may be positioned with computer-controlled mechanicalactuators 110, 135, in a manner similar to that utilized by CAM machinetools. There are many mechanical systems that may accomplish therequired motion, using a combination of electric, hydraulic or pneumaticmotors and linear actuators, belts, pulleys, lead screws, and otherdevices. In one embodiment, the metal wire electrode 120 is situated ona gantry system 105 that allows motion in the X and Y directions, asdescribed above. The baseplate electrode 130 moves independently on theZ axis. The feed of metal wire 120 may be controlled by anotherindependent actuator controlling source 125. The timing, duration, andpower of the electric current used for deposition are controlled bycontroller 145. The formation of a structure, controlled by signals fromcontroller 145, may proceed according to the following example. Thestructure is a simple cube, formed from eight particles each having adiameter of 1 unit.

1. The gantry 105 moves wire 120 to the first position (X0,Y0) in the XYplane.

2. The baseplate 130 moves to a position close to the tip of the metalwire 120 in the Z axis (Z0).

3. Wire 120 is fed from source 125 until it contacts the baseplate 130.

4. Electric current flows through the electrodes (i.e., the baseplate130 and wire 120), melting the tip of the wire 120 and forming a metalparticle on the baseplate 130.

5. The gantry 105 moves the wire 120 to the next position in the XYplane (X1,Y0).

6. Wire 120 is fed to contact the baseplate 130, current is passed, andanother particle is formed.

7. The gantry 105 moves the wire 120 in the XY plane and forms two moreparticles at X1,Y1 and X0,Y1.

8. The baseplate 130 moves one unit away from the metal wire 120 (Z1).

9. The gantry 105 moves the wire 120 to (X0,Y0), wire 120 is fed fromsource 125 until it makes contact with the particle underneath, and anew particle is formed on top of the previously deposited particle.

10. The gantry 105 moves the wire 120 to each remaining XY positionagain in order, depositing a particle at each on top of the previouslayer.

Like many CAM tools, the metal-based additive manufacturing process inaccordance with embodiments of the present invention may be combinedwith other tools and/or processes in a single machine. Examples of thisare a gantry-type machine as described above with a polymer extrudertool and a milling cutter tool attached to the gantry alongside themetal deposition tool. In this manner, hybrid structures may be builtfrom a combination of polymer and metal, using the combination toincrease the speed of building the structure, reduce the cost of thestructure, or using the material that has the desired properties forthat portion of the structure. For example, a part fabricated inaccordance with embodiments of the present invention may have astructure that is largely built from a non-conductive polymer but thatalso features internal printed metallic electric circuits. The millingcutter may be used to machine any precision surfaces required on thestructure. This concept may be expanded to include any number of toolsin a single machine to perform any operation required for the formationof the required structure.

Multiple parts may be produced in succession in an automated fashionwith no human user involvement. After a part is complete, an arm maycross the baseplate 130 and remove the part, depositing it into acollection area. Once the baseplate 130 is cleared of the previous partand the removal arm, the next part may be fabricated.

In some embodiments of the present invention, calculations for thedeposition parameters performed by 3D rendering module 155 are based ona static diameter value for the metal wire or polymer filament. However,the diameter of the supplied filament may be variable, as describedabove, and these variations may cause poor printing performance,jamming/clogging of the wire feeder 115 (e.g., a nozzle), or in severecases damage to mechanical systems of apparatus 100. It may also bedesirable to detect the absence of wire 120 to determine when the source125 has been exhausted. Additionally, a precise measure of the absolutelength of wire 120 consumed may be logged and used to develop algorithmsto better project the total wire 120 required and the time to complete aprint.

In various embodiments of the present invention, in order to sense andtrack the use of wire 120 (or its absence), the apparatus 100incorporates a system that includes or consists essentially of either amechanical wheel that is in contact with the wire 120, or an opticalsystem that has an unimpeded view of the 120. FIG. 6 schematicallydepicts a mechanical wire-tracking system 600 that includes a wheel 610that contacts the wire 120 at a point within the wire feeder 115 as thewire 120 is fed from source 125 during printing. The motion of the wire120 may be recorded by a digital encoder connected to the wheel 610. Theamount of wire 120 utilized during a period of time may be calculatedfrom the encoder readout. As shown, the wheel 610 may be connected to amechanism such as a spring-loaded lever 620 that urges the wheel 610against the wire 120. In this manner, deflections of the lever 620 maybe used to calculate the diameter of the wire 120. Absence of wheelmotion or a very small diameter measurement will typically indicate thatthe source 125 has been emptied of wire 120.

FIG. 7 depicts an optical wire-tracking system 700 that may beincorporated into various embodiments of the present invention. Anoptical image sensor 710 may be utilized to determine movement in of thewire 120 based on microscopic changes in the wire's surface andtherefore be used to measure absolute length of wire 120 utilized duringa printing process. A light 720 angled on the backside of the wire 120facing the sensor 710 may be used to measure the diameter of the wire120 based on the area of light blocked by the wire 120. Multiple sensors710 may be used to provide more accurate measurements in multiple axeswith respect to the wire 120. Similarly to the wire-tracking system 600,the motion and diameter of the wire 120 may be used to calculate totallength of wire utilized, detect when source 125 is out of wire, etc.

Printers in accordance with embodiments of the present invention mayalso incorporate an anti jamming mechanism to prevent drasticallyoversized wire from causing a jam or other damage to the wire feeder(e.g., the nozzle thereof). For example, a ring having an insidediameter matching the maximum allowable wire diameter may be disposedwithin the wire feeder 115 or between the wire feeder 115 and the source125. The wire 120 may be passed through the ring, and if it isoversized, the wire may become stuck in the ring or otherwise be unableto pass through the feeder 115 for printing. This condition may besensed by, e.g., wire-tracking system 600 or 700, and reported to theoperator. Additionally, FIG. 8 depicts an embodiment of such a ring 800.As shown, ring 800 may have a sharp edge on the inner diameter so thatthe wire 120 may be automatically trimmed to the proper diameter as itpasses through the ring 800.

Some printed parts, particularly those having high densities and/orvariable or complicated geometries, may be difficult to remove from thebaseplate 130 after printing. In various embodiments of the invention, asacrificial structure (or “raft”) may be printed on the baseplate 130before the part and utilized to enable removal of the part from thebaseplate 130. In various embodiments, the structure of the raft isselected to facilitate anchoring of the part to the baseplate 130 andenable electrical conductivity between the part (i.e., the wireelectrode) and the baseplate 130 while facilitating removal of the raftfrom the finished part after printing. Furthermore, rafts having thesame size and/or shape and/or interior configuration may be utilized forparts having very different geometries, thereby enabling a standardizedprocess for removal of different parts from the baseplate 130—afterprinting, the raft (and the printed part thereover) is removed from thebaseplate 130, and then the raft is removed from the part. In variousembodiments, the raft may include, consist essentially of, or consistof, e.g., metal and/or polymer. In various embodiments, the raft is notprinted by the apparatus 100 but is provided by other means (e.g.,fabricated by another apparatus and affixed (e.g., adhered) to thebaseplate 130 prior to printing of a desired part). In variousembodiments, the raft includes, consists essentially of, or consists ofone or more materials different from that utilized to fabricate a partthereon. For example, wires including, consisting essentially of, orconsisting of different metals may be utilized to print the raft and toprint one or more parts thereover.

FIGS. 9A and 9B are schematic top views of rafts 900 fabricated inaccordance with embodiments of the present invention. As shown, the raftmay include or consist essentially of one or more layers of materialprinted (e.g., using wire 120) over the baseplate 130 before printing ofthe desired part. In order to facilitate subsequent removal of the raftfrom the printed part, the raft may be composed of, e.g., a series ofstripes 910 or a grid pattern 920 of the printed material, as shown inFIGS. 9A and 9B. That is, in various embodiments, the raft 900 definesone or more openings therethrough that extend between the baseplate 130and a part printed over the raft 900, rather than the raft 900 beingcomposed of a solid sheet of material. The raft 900 may be printedutilizing a wire 120 that corresponds to the wire 120 (i.e., the samematerial and/or the same wire diameter and/or deposition conditions)utilized to print the part over the raft 900, or the raft 900 may beprinted utilizing a different material, different wire diameter, and/ordifferent deposition conditions (e.g., wire withdrawal rate).

In various embodiments, the raft 900 is at least partially composed ofprinted areas having thicknesses 930 with gaps 940 therebetween. Thesizes of thicknesses 930 and/or gaps 940 may be selected to control theadhesion between the raft 900 and the printed part and/or the baseplate130. Instead or in addition, the height (i.e., vertical thickness) ofall or a portion of the raft 900 may be selected to facilitatesubsequent printing of a part thereover. FIG. 9C depicts a part 950printed over an exemplary raft 900 composed of one or more bottom layers960, one or more middle layers 970, and one or more top layers 980. Thebottom layer 960 may have a thickness greater than the layer thicknesstypically utilized for printing parts in order to, e.g., isolate thepart from any roughness or unevenness of the surface of the baseplate130. For example, if printed parts are typically composed of layershaving thicknesses of approximately 0.6 mm, then at least the bottomlayer 960 of the raft 900 may have a thickness greater than 0.6 mm,e.g., greater than 1 mm, or even thicker. The exemplary raft 900 in FIG.9C also contains one or more middle layers 970 that typically do notmechanically contact either the baseplate 130 or the part 950. Themiddle layer(s) 970 may, for example, provide structural stability tothe raft 900 while also providing electrical conductivity through theraft 900. The top layer 980 may have a structure designed to control theamount of adhesion between the raft 900 and part 950 printed over theraft. For example, the porosity of top layer 980 and/or the size of gaps940 of the top layer 980 may be increased to decrease the amount ofsurface area at the interface (and thus the adhesion) between the raft900 and the part 950.

Once the part 950 has been printed as detailed herein, the part 950 andthe raft 900 may be separated from the baseplate 130. FIG. 10Aillustrates an exemplary embodiment in which a blade 1000 is utilized toseparate the raft 900 from the baseplate 130. As shown in FIG. 10B,after separation of the raft 900 from the baseplate 130, the raft may bepeeled away from the part 950.

In accordance with various embodiments of the invention, the printingapparatus 100 may be a single “station” along an assembly line ofmodular automated manufacturing stations in order to leverage theautomation capabilities of apparatus 100. For example, a part may beprinted utilizing an apparatus 100 and then automatically transferred(via, e.g., a conveyor belt, robotic handler, or similar system) to afinishing station (e.g., rock tumbler, vibration box, bead blastingcabinet, etc.) and thence to a cleaning station for automaticsterilization with UV light, chemicals, etc. The part may then betransferred into, e.g., a plastic wrap station, and then to a packagingstation with an automatic labeler that labels the boxed parts as theyexit. A parallel assembly line may produce packing material for theprinted part. For example, a mold of the printed part may be utilized toshape packaging foam such that it is form-fitted to the finished part.The shaped foam may be fed into the packaging system along with a box inthe main assembly line.

In accordance with various embodiments of the invention, wire-trackingsystems such as wire-tracking systems 600, 700, as well as rafts (e.g.,raft 900) and/or other portions of apparatus 100 may be utilized withwires composed of non-metallic materials (e.g., plastic) and/or to printnon-metallic (e.g., plastic) objects.

The terms and expressions employed herein are used as terms andexpressions of description and not of limitation, and there is nointention, in the use of such terms and expressions, of excluding anyequivalents of the features shown and described or portions thereof. Inaddition, having described certain embodiments of the invention, it willbe apparent to those of ordinary skill in the art that other embodimentsincorporating the concepts disclosed herein may be used withoutdeparting from the spirit and scope of the invention. Accordingly, thedescribed embodiments are to be considered in all respects as onlyillustrative and not restrictive.

1. A system for printing at least a portion of a three-dimensional (3D)object adjacent to a support, comprising: a source of at least onefeedstock; a support for supporting said at least said portion of said3D object during formation; a feeder for directing said at least onefeedstock from said source towards said support; a power supply forsupplying electrical current through said at least one feedstock andinto said support; and a controller operatively coupled to said powersupply, wherein said controller: (i) receives in computer memory acomputational representation of said 3D object, (ii) subsequent toreceiving said computational representation of said 3D object, directssaid at least one feedstock through a feeder towards said support, (iii)upon directing said at least one feedstock through said feeder, directsflow of electrical current flow through said at least one feedstock andinto said support, (iv) subjects said at least one feedstock to Jouleheating upon flow of electrical current through said at least onefeedstock and into said support, which Joule heating is sufficient tomelt at least a portion of said at least one feedstock, such that saidat least said portion of said at least one feedstock deposits adjacentto said support, thereby printing said at least said portion of said 3Dobject in accordance with said computational representation of said 3Dobject.
 2. The system of claim 1, wherein said feeder comprises anopening for directing said at least one feedstock towards said support.3. The system of claim 2, wherein said at least one feedstock isdirected from a spool through said opening.
 4. The system of claim 1,wherein said Joule heating is sufficient to melt only said portion ofsaid at least one feedstock.
 5. The system of claim 1, wherein saidcontroller directs deposition of additional portion(s) of said at leastone feedstock adjacent to said support by repeating (iv) one or moretimes.
 6. The system of claim 1, wherein said controller directs anadditional feedstock through said feeder and subjects said additionalfeedstock to Joule heating, such that at least a portion of saidadditional feedstock deposits adjacent to said support or adjacent tosaid at least said portion of said at least one feedstock.
 7. The systemof claim 1, wherein said controller selects a size of said portion ofsaid 3D object by controlling a speed of retraction of said at least onefeedstock.
 8. The system of claim 1, further comprising one or moremechanical actuators to change a relative position of said at least onefeedstock and said support after deposition of said at least saidportion of said 3D object.
 9. The system of claim 1, wherein saidcontroller comprises said computer memory and a 3D rendering module,wherein said computer memory stores a computational representation ofsaid 3D object and said 3D rendering module extracts sets of datacorresponding to successive voxels or layers from said computationalrepresentation.
 10. The system of claim 1, wherein said at least onefeedstock comprises a plurality of feedstocks, and wherein saidplurality of feedstocks comprises a plurality of different metals. 11.The system of claim 1, wherein said at least one feedstock comprises oneor more elements selected from the group consisting of stainless steel,copper, and aluminum.
 12. The system of claim 1, wherein said electricalcurrent is subjected to flow through said at least one feedstock andinto said support using said power supply that is in electricalcommunication with said at least one feedstock.
 13. The system of claim1, wherein said power supply is in electrical communication with said atleast one feedstock through said feeder, and wherein said power supplyis in electrical communication with said support.
 14. The system ofclaim 1, wherein during use, said at least said portion of said 3Dobject is formed in response to heat arising from, at least in part,contact resistance between said at least one feedstock and said 3Dobject or said support.
 15. The system of claim 1, further comprising afeedback control unit that measures a deposition parameter and/orcharacteristic of said at least said portion of said at least onefeedstock, wherein said controller prints said at least said portion ofsaid 3D object in accordance with said deposition parameter and/orcharacteristic.
 16. The system of claim 15, wherein said feedbackcontrol measures one or more of (i) contact resistance, (ii) voltage,(iii) current, (iv) temperature of said support, (v) temperature of saidat least said portion of said at least one feedstock, (vi) temperatureof said at least said portion of said 3D object, (vii) amount of said atleast said portion of said at least one feedstock, (viii) dimensions ofsaid at least said portion of said at least one feedstock, (ix) movementof said at least said portion of said at least one feedstock, (x) damageduring deposition, (xi) speed of deposition, (xii) heat duringdeposition, (xiii) spacing among individual portions of said at leastsaid portion of said 3D object, (xiv) spacing between said at least saidportion of said at least one feedstock and said support, (xv) spacingbetween said at least said portion of said at least one feedstock andsaid at least said portion of said 3D object, and (xvi) porosity of saidat least said portion of said 3D object.
 17. The system of claim 15,wherein said controller includes said feedback control unit.
 18. Thesystem of claim 1, wherein said controller directs formation of asacrificial raft structure prior to printing said at least said portionof said 3D object, which sacrificial raft structure (a) anchors said atleast said portion of said 3D object to said support and (b) permitsremoval of said at least said portion of said 3D object from saidsupport.
 19. The system of claim 18, wherein at least one of a densityand a porosity of said sacrificial raft structure is less than that ofsaid 3D object.
 20. The system of claim 18, wherein said sacrificialraft structure comprises a plurality of layers.
 21. The system of claim18, wherein a thickness of at least one layer of said sacrificial raftstructure is greater than a thickness of at least one layer of said 3Dobject.
 22. The system of claim 18, wherein said sacrificial raftstructure and said 3D object are formed from different materials. 23.The system of claim 18, wherein subsequent to printing said at leastsaid portion of said 3D object, said controller directs (i) removal ofsaid sacrificial raft structure from said support, and (ii) separationof said sacrificial raft structure from said at least said portion ofsaid 3D object.